Independent work № 9

LAW OF ENERGY CONSERVATION

Option 1

First level


  1. The ball is thrown vertically upwards. Assuming that air resistance can be neglected, choose the correct statement.

    1. The momentum of the ball as it rises remains constant.

    2. When lifting the ball kinetic energy turns into potential.

    3. The total mechanical energy of the ball increases as it rises.

  2. The speed of a freely falling body with a mass of 0.5 kg increased from 2 m/s to 4 m/s. Assuming that air resistance can be neglected, choose the correct statement.

    1. The momentum of the body has doubled.

    2. The kinetic energy has doubled.

    3. The total mechanical energy of the body decreased by 2 times.




Average level

  1. Which of the following bodies have potential energy: a) a ball rolling on the ground; b) a bow with a stretched string; c) gas compressed in a cylinder; d) a Ferris wheel booth?

  2. Find the potential energy of a body of mass 500 g raised to a height of 2 m from the surface of the earth.
Enough level

  1. Find the potential and kinetic energy of a body of mass 3 kg, falling freely from a height of 5 m, at a distance of 2 m from the surface of the earth.

  2. With what speed must a ball be thrown down from a height of 5 m in order for it to bounce to a height of 10 m? The impact on the ground is absolutely elastic. Ignore air resistance.
Independent work No. 9

LAW OF ENERGY CONSERVATION

In other words, it gains kinetic energy and loses potential energy. Then it's a shock between your ball and the ground. If all the energy is transferred to the ground during this hit, the ball will not bounce. On the other hand, if the energy is held by the ball after the impact, then it will bounce.

Finally, lose the bouncing ball, it will bounce even more than a tennis ball! We have just seen that all this is due to the conservation of some energy during the shock. If some balls have the ability to bounce more than others, i.e. store more energy than others, it is due to their structure.

Option 2

First level










  3. Two balls of masses 1 kg and 2 kg move in a horizontal plane at right angles to each other with velocities equal to 2 m/s and 1 m/s, respectively. Choose the correct statement.

    1. The kinetic energy of the first ball is equal to the kinetic energy of the second ball.

    2. The momentum of the first ball is less than the momentum of the second ball in absolute value.

    3. The sum of the kinetic energies of the balls is greater than 2.5 J.
Average level

  1. Which of the following bodies have kinetic energy: a) a stone raised above the ground; b) a flying plane; in) stretched spring; d) a flying balloon

  2. A stone thrown from the surface of the earth at a speed of 10 m/s had a kinetic energy of 5 J at the top of the trajectory. Determine the mass of the stone.
Enough level

  1. A plane taking off, rising to a height of 10 km, picks up a speed of 250 m/s. Compare the kinetic and potential energies acquired by the aircraft: which one is greater and by how many times?

  2. A body is thrown vertically upward with a speed of 20 m/s. At what height will its kinetic energy equal its potential energy?

Independent work No. 9

Indeed, the ball is elastic, that is, the material that represents it can be deformed. In addition, the inside of the balloon contains air, which is also elastic. You don't see it during the kick, but the ball flattens against the ground. Inside the air then has less space: it is compressed. Then, a little like a spring, the air will relax and the ball will resume its round shape by tapping the ground: it's rebound!

Here is a short video that illustrates my point. A golf ball is thrown at a very high speed onto a steel plate. It completely deforms during shock, but then gradually resumes its original shape. Bouncing balls have much higher rebounds than other balls because they are made from special materials, polymers, which make them very resilient. During ground shock, they do not deform and therefore lose very little energy. Rubber is the material used to make these "superballs" you'll find on the market, but other elements such as wood glue and borax can also be used.

LAW OF ENERGY CONSERVATION

Option 3

First level


  1. An icicle falls from the roof of a house. Assuming that air resistance can be neglected, choose the correct statement.

    1. The potential energy of the icicle at the end of the fall is maximum.

    2. The kinetic energy of an icicle does not change as it falls.

    3. The total mechanical energy of the icicle is conserved.

  2. A car travels along a horizontal ring road with a constant modulo speed. Choose the correct statement.

    1. The potential energy of the car is reduced.

    2. The momentum of the car only changes in direction.

    3. The kinetic energy of the car increases.

  3. The boy threw a stone into the sea. Assuming that the stone in the water moves uniformly vertically, choose the correct statement.

    1. The total mechanical energy of the stone does not change.

    2. The kinetic energy of the stone increases.

    3. The momentum of a pebble does not change when moving in water.
Average level

  1. At the same height are a piece of marble and a piece of lead of the same volume. Which of these bodies has the highest potential energy?

  2. A ball of mass 400 g rolls on a horizontal table at a speed of 15 cm/s. What is its kinetic energy?
Enough level

  1. Find the potential energy of a body of mass 500 g thrown vertically upwards at a speed of 10 m/s at the highest point of the ascent.

  2. A body of mass 10 kg falls freely from a height of 20 m from rest. At what height is the kinetic energy three times the potential energy?

>>Physics: Law of Conservation of Energy

The ball will not bounce forever. At some point, his movement will stop. Indeed, with each impact with the ground, the ball loses a little energy, and the bounces are less and less. In a memorable scene from the movie, Indiana Jones and the Raiders of the Lost Ark have Indy running down a hallway in front of huge stone balls.

From what the minimum height was to lay the stone balls, which consider the kinetic energy acquired? Consider first that the balls roll right on the corridor floor. Then solve the problem for the situation where the ball is rolling into the trough, as shown in Fig. Suppose the ball touches the binding at a perpendicular distance \\ from the center.

The term "energy" was introduced in 1807 by the English scientist T. Jung. Translated from Greek, this word means In the general case, the body has both kinetic and potential energy at the same time. Their sum is called full mechanical energy:
E \u003d E k + E p (15.1)
This concept was introduced in 1847 by the 26-year-old German scientist G. Helmholtz.

Note: Comments should be understood that, in fact, the balls lose the mechanical energy of friction and deformation of the stalactites. The radius of the sphere will be equal to 1 m rolling in the ditch. . We talked about the law of conservation of energy in a perfectly flexible collision and that in a real collision some of the deformation energy is converted into heat and that, for example, a ball after bouncing off the ground rises to the height from which it was launched. How do we explain the next attempt then?

Attempt: Take a light and heavy ball. We hold two balls at a certain height above the table so that the ping pong ball rests on a heavier rubber ball and in this position we drop the balls onto the table. The balls must hit the table vertically. A ping pong ball flies after bouncing up to several times the height of two balls.

What happens to complete mechanical energy as the body moves? To find out, consider a simple phenomenon.

Throw the ball vertically upwards. By giving the ball speed , we thereby give it some kinetic energy. As the ball moves upward, its movement will be slowed down by the gravity of the Earth and the speed, and with it the kinetic energy of the ball, will become less and less. The potential energy of the ball, along with the height k, will increase in this case. At the highest point of the trajectory (at maximum height) potential energy the ball reaches its the greatest value, and the kinetic energy will be zero. After that, the ball will begin to fall down, gradually picking up speed. In this case, the kinetic energy will begin to increase, and the potential energy (due to a decrease in height) will decrease. At the moment of hitting the ground, the kinetic energy of the ball will reach its maximum value, and the potential energy will turn to zero.

Doesn't this contradict the law of conservation of energy? And can we determine how high a ping pong ball is? comment. On the first question, if there is no violation of the energy saving law when trying two balls, we will once again answer in detail the result of the experiment. The conservation law will be violated if not only does a light ball fly higher than its original position, but if at the same time a lower heavier ball flies at least to the same height as at the beginning.

However, when we try, we can see that the heavier the ball rises a lot less. Therefore, we can assume that the heavier ball transferred some of its energy to the lighter ball in the collision, which is why the conservation law is so "saved". How big is the amount of energy transferred between the balls, we will say later and try to answer the second question - how high will ping-pong fly.

So, when the kinetic energy of the body decreases, the potential energy increases, and vice versa, when the kinetic energy of the body increases, its potential energy decreases. The study free fall body (in the absence of air resistance) shows that any decrease in one of these types of energy is accompanied by an equal increase in the other type of energy. The total mechanical energy of the body is conserved in this case. This is what law of conservation of mechanical energy:

The problem will be simplified if we first assume that the weight of the ping-pong ball is negligible by the weight of the heavier ball, and that the weight of the two balls is negligible relative to the table they land on. We can then use the results of setting the ball's speed at the foot of the ball. Similarly, the amount of change in speed of a ping pong ball after a heavy ball is 2 inches, but this change in speed is not towards the table, but towards the heavier ball. If the size of a ping-pong ball after bounce is three times greater than before impact, the height at which the ball will fly is nine times the height at which the balls were dropped.

The total mechanical energy of a body, which is not affected by the forces of friction and resistance, remains unchanged during its motion.

If we designate the initial and final energies of the body as E and E", then the law of conservation of energy can be expressed as the following equation:
E = E" (15.2)
Let us assume that a freely moving body at the initial moment of time was at a height hо and had a speed V 0 . Then its total mechanical energy at this moment of time was equal to:

The actual height at which a ping pong ball flies can be measured simply by holding the balls next to the balls perpendicular to the table, dropping the balls from a height such as 20 cm above the table, and observing the high ping pong ball. We will find that it will only fly so many times, depending on which balloons we are using.

How is it possible that the actual height at which the ball flies is so different from the theoretical result? It would be interesting to see how this difference is caused, namely, how the change in the speed of a body during a collision depends on the material of the body and on its weight.

If after some time the body under consideration is at a height h, having a speed V(Fig. 28), then its total mechanical energy will become equal to:



According to the law of conservation of energy, both these values ​​of energy must match. That's why

If the initial values ​​h 0 and are known, then this equation allows you to find the speed of the body V at height h or, conversely, height h, at which the body will have a given speed V. In this case, the mass of the body will not play any role, since in equation (15.5) it is reduced.

It should be remembered that the total mechanical energy is conserved only when there are no friction and resistance forces. If these forces are present, then their action leads to a decrease in mechanical energy.

??? 1. What is called total mechanical energy? 2. Formulate the law of conservation of mechanical energy. 3. With what energy - kinetic or potential - does the total mechanical energy of a freely falling body coincide with the moment it hits the ground? 4. What energy coincides with the total mechanical energy of a ball thrown vertically upward at the moment when it is at the highest point of its flight? 5. What happens to the total mechanical energy of the body in the presence of friction and resistance forces?

S.V. Gromov, N.A. Motherland, Physics Grade 8

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